What are View Coordinates?
When you switch to View space Coordinates, all transformations depend on the view; So the x axis is horizontal and y is vertical; If you move your view, so move the axis.

It's useful if you only want to move a particular object relative to the view.

To lock a transform to any axis, while in View transform orientation, press the respective axis key twice; So to lock translate to the View's X axis, press G X X.